Phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylglycerol (PG) and diphosphatidylglycerol (DPG) are the major phospholipids found in plant tissues. The distribution of these lipids among the various organelles of different tissues and among different plants has been comprehensively studied. The pathways by which these lipids are synthesized have also been studied extensively but very few of the plant enzymes involved in these pathways have been purified or their corresponding genes cloned.
The precursor molecule common to the de novo synthesis of all phospholipids in prokaryotes and eukaryotes is phosphatidic acid (PA). Synthesis of PA occurs by the sequential acylation of glycerol-3-phosphate by glycerol-3-phosphate acyltransferase and 1-monoacylglycerol-3-phosphate acyltransferase, both of which utilize acyl-CoA as a source of acyl moieties. PA may be converted to CDP-diacylglycerol by the action of the enzyme CDP-diacylglycerol synthase (E.C. 2.7.7.41; also called CTP:phosphatidate cytidylyltransferase, phosphate cytidylyltransferase, phosphoethanolamine cytidylyltransferase, among others). This enzyme has been characterized in yeast where it has been demonstrated to be highly regulated (Homann et al. (1987) J. Bacteriol. 169:3276-3280). While phosphatidate cytidylyltransferase activity has been detected in the chloroplast, mitochondria and microsomes of several plants, no sequence information of plant phosphatidate cytidylyltransferase has been confirmed. The sequence of an Arabidopsis thaliana putative phosphoethanolamine cytidylyltransferase was identified when the sequence of the chromosome 2 was determined (Lin et al. (1999) Nature 402:761-768).
In castor bean endosperms PE is sequentially methylated to PC by methyltransferases which utilize S-adenosylmethionine as the methyl donor. PE N-methyltransferase (EC 2.1.1.17) catalyzes the methylation of PE to phosphatidyl methylethanolamine (PME) and phosphatidyl-N-methylethanolamine N-methyltransferase (EC 2.1.1.71; also called phosphatidylethanolamine N-methyltransferase) catalyzes the two methylations necessary to convert PME to PC (McGraw and Henry (1989) Genetics 122:317-330). The sequence of a plant phosphatidylethanolamine N-methyltransferase has yet to be determined.
Identification of the sequences encoding phosphoethanolamine cytidylyltransferase or phosphatidylethanolamine N-methyltransferase in plants will allow the manipulation of these genes in transgenic plants.